The present invention relates to an image pickup apparatus (also referred to as the imaging apparatus) and a method of controlling the same.
In accompanying remarkable progress in the digital signal processing techniques, there have been developed and realized numerous and various functions in the field of the image pickup apparatus typified by a video camera. Further, the image pickup apparatus such as video camera is now attracting attention as a picture or image input apparatus for a computer or the like image processing system because of capability for easily making available digital video signals. Data of the picture or image handled by the computer or the like system is in general those of still pictures. Accordingly, in order to obtain a video signal for a still picture, it is generally practiced to extract a video signal of a given one field or frame a motion picture signal outputted from a camera-integrated type video tape recorder for domestic use, wherein the extracted video signal is stored once in a memory or storage device. The video signal recorded in the memory or the like storage medium is then inputted to the computer as still picture data. In this conjunction, a technique for generating a still picture with correct exposure with a video camera is disclosed in JP-A-2-288679 (Japanese Unexamined Patent Application Publication No. 288679/1990).
However, in the conventional image pickup apparatus or video camera, the image sensor device (also referred to as the imager or charge coupled device, i.e., CCD in abbreviation) allows the signal charges to be read out only once from the pixels (picture elements) of the image sensor device constituted by photodiodes with color filters. In other words, only the so-called destructive read-out operation is permissible. Besides, it is required to read out mixedly the signals simultaneously from two pixels located adjacent to each other in the vertical direction (this read-out scheme is referred to as the two line mixed read-out scheme). With such read-out scheme, however, it is impossible to generate a still picture having a resolution comparable to the number of pixels arrayed in the image sensor device.
As another hitherto known image pickup apparatus capable of generating both a still picture and a motion picture, there may be mentioned an image pickup apparatus disclosed, for example, in JP-A-59-194575 (Japanese Unexamined Patent Application Publication No. 194575/1984), in which the still picture pickup operation is performed by opening and closing an iris diaphragm.
However, most of the iris diaphragms employed in an imaging devices or the video cameras destined for domestic use is implemented in a simple structure incorporating a meter-type diaphragm blade driving mechanism in which a coil employed usually in meters such as DC meters and a permanent magnet are so combined that repulsion/attraction produced between a magnetic field generated by a current flowing through the coil and the permanent magnet are utilized for setting a specific iris value through cooperation of resiliency imparted to the blades constituting the diaphragm.
The diaphragm device of the structure mentioned above is not inherently designed for use in the application where a high-speed operation thereof is required. However, in order to operate the diaphragm of this type at a high speed, it becomes necessary to additionally provide a high-speed servo-loop with a view to increasing the response performance.
Thus, with the meter-type diaphragm device of the structure mentioned above, difficulty will be encountered in operating the diaphragm at a high speed which is required for generating a still picture (i.e., for realizing a still picture pickup mode) with satisfactory picture quality as described in the above publication. Such being the circumstances, in the still picture pickup operation, it is practiced in most cases to provide a mechanical shutter capable of performing open/close operation at a high speed in addition to the meter-type diaphragm device.